National Institute of Materials Physics Bucharest
National Institute of Materials Physics Bucharest
Birsan A.,National Institute of Materials Physics Bucharest
Journal of Alloys and Compounds | Year: 2017
The structural electronic and magnetic properties of CoFeZrSi material with tetragonal and triclinic distorted primitive cell were investigated within the Density Functional Theory (DFT) framework. Half-metallic soft ferromagnetism with high Curie temperature was evidenced in this quaternary compound which crystallizes optimally in the cubic LiMgPdSn type structure. The material preserves the half-metallic characteristic when the cubic primitive cell is tetragonally deformed or when the unit cell volume increases or decreases due to external applied pressure. On contrary, the energy band gap, typical for half-metals, located in the minority spin channel disappears when the unit cell is triclinic deformed, e.g. under interfacial distortions. The material exhibits metallic behavior and the total magnetic moment increases from the integral 1μB/f.u., calculated for cubic and tetragonal unit cells, up to 3.25 μB/f.u. for triclinic structure with a = b = c and α=β=90o, γ=89o. © 2017 Elsevier B.V.
Polosan S.,National Institute of Materials Physics Bucharest
Journal of Non-Crystalline Solids | Year: 2017
Dielectric measurements of bismuth germanate oxides reveal significant changes in the amorphous phase, between 200 and 350°C, where the crystallisation processes start below the glass transition temperature. The capacitive and loss measurements versus temperature and frequency suggest an incipient glass transition below 350°C associated with an increase in the clusters, mainly those formed by GeO4 tetrahedra, responsible for the dipolar orientation effects. An increase in the capacitive parameter versus temperature at lower frequencies, especially over 250°C has been associated with the increase in mobility of the clusters. The physical meaning of those processes has been associated with the formation of a highly viscous layer enriched in GeO4 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth. A higher frequency is required in the crystallisation processes to compensate for the thermal disorder in the amorphous materials. The crystallisation process is identified by the decrease in the dielectric constant despite the increase in temperature. © 2017 Elsevier B.V.
Gartner P.,University of Bremen |
Gartner P.,National Institute of Materials Physics Bucharest
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011
The problem of the two-level laser is studied analytically. The steady-state solution is expressed as a continued fraction and allows for accurate approximation by rational functions. Moreover, we show that the abrupt change observed in the pump dependence of the steady-state population is directly connected to the transition to the lasing regime. The condition for a sharp transition to Poissonian statistics is expressed as a scaling limit of vanishing cavity loss and light-matter coupling, κ→0, g→0, such that g2/κ stays finite and g2/κ>2γ, where γ is the rate of nonradiative losses. The same scaling procedure is also shown to describe a similar change to the Poisson distribution in the Scully-Lamb laser model, suggesting that the low-κ, low-g asymptotics is of more general significance for the laser transition. © 2011 American Physical Society.
Moldoveanu V.,National Institute of Materials Physics Bucharest |
Tanatar B.,Bilkent University
Physical Review B - Condensed Matter and Materials Physics | Year: 2010
The effect of Rashba spin-orbit coupling on the spin interference in a noninteracting one-dimensional ring connected to two leads is studied theoretically within the nonequilibrium Greens' function formalism. We compute the charge and spin currents and analyze their Aharonov-Bohm oscillations. The geometry of the system is conveniently described by the angle δ between the two leads. We show that for δ=180°(i.e., for symmetrically coupled leads), a good filtering of up- or down-spin orientation is obtained around half-integer multiples of Φ/ Φ0. These particular flux values are degeneracy points for clockwise and counterclockwise propagating states, corresponding to the same spin orientation in the local spin frame of the ring. In contrast, for the asymmetric coupling, i.e., δ=135°, the filter efficiency is maximum around integer multiples of Φ/ Φ0. The numerical results suggest that the spin filtering is obtained when the clockwise or counterclockwise states interfere destructively. It turns out that the spin filtering regime is stable against variations in the bias applied on the system. The quasiperiodic oscillations of the charge current, as a function of the Rashba strength, are obtained and discussed. © 2010 The American Physical Society.
Miu L.,National Institute of Materials Physics Bucharest
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the pinning barriers by thermal fluctuations. In the case of YB 2Cu 3O 7 theoretical estimates give a depinning temperature T dp very close to the critical temperature T c (T dp ∼ 0.95T c), while the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T dp (∼0.5T c). We determined the temperature T variation of the normalized magnetization relaxation rate S for YB 2Cu 3O 7 films containing BaZrO 3 nanorods preferentially oriented along the c axis, with the external magnetic field applied along the nanorods. By extending the T interval up close to T c, below the matching field a rich nonmonotonous S(T) variation was observed. It is shown here that the often analyzed S(T) maximum occurring at relatively low T (which was connected to a disappointing T dp) has an extrinsic origin, related to thermomagnetic instabilities. The accommodation of vortices to the columnar pins in the presence of the T-dependent macroscopic currents induced in the sample during experiments is actually signaled by a pronounced S(T) deep located at high T, indicating that T dp remains close to T c, in agreement with the theoretical prediction. © 2012 American Physical Society.
Mihalache V.,National Institute of Materials Physics Bucharest |
Pasuk I.,National Institute of Materials Physics Bucharest
Acta Materialia | Year: 2011
CeO2 films were prepared from solutions of different concentrations (0.05-1.0 M) on textured Ni substrates. Homogeneous nucleation and growth of CeO2 nanocrystals <8 nm occurs upon calcination at 350-500 °C. At the heating and sintering stage, the homogeneous growth is inhibited in favor of the development of grains with (0 0 l) texture. The grain growth normal to the film surface is well described by a stretched exponential function with a relaxation time of up to 60 min and with Kohlrausch exponent values of less than unity. The increase in grain size is accompanied by the relaxation of the microstrain. During the relaxation time, the grain coarsening is controlled by surface diffusion characterized by an activation energy as low as 0.6 eV. At the relaxation time, the surface morphology is strongly concentration dependent. The surface morphology changes from separated (agglomerated) grains to a continuous grain configuration as the concentration increases from 0.05 M to 0.8 M. After the relaxation time, both the grain size normal to the film surface and the lateral grain size continue to grow, and the grain configuration continues to change. These processes are concentration dependent. In the films with a nominal thickness >170 nm (>0.8 M), the transition to classical curvature-driven grain-growth kinetics is evident (at 1000 °C, below the literature value of 1100 °C). The decrease in the Kohlrausch exponent for these thick films suggests that the grain coarsening through grain boundary migration is responsible for the stretched regime of grain-size relaxation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Secu M.,National Institute of Materials Physics Bucharest
Journal of Nanoparticle Research | Year: 2011
Oxyfluoride glass-ceramic in the system SiO2-Al 2O3-CaF2-SmF3 containing Sm 3+-doped CaF2 nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm3+-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 °C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm2+-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 °C were attributed to the recombination of the electrons released from the Sm2+-traps located mainly at the surface of the CaF2 nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 °C size have been assigned to the recombination of the electrons released from the Sm2+-traps located inside the CaF 2 nanocrystals. © Springer Science+Business Media B.V. 2010.
Sandu V.,National Institute of Materials Physics Bucharest
Modern Physics Letters B | Year: 2012
We present the scaling law of the pinning force F P in intermediate and high temperature superconductors, the factors which influence its field and temperature dependence, the limitations imposed by the enlarged anisotropy and high operating temperatures, giant creep included. The theoretical developments which incorporate the new features of these classes of superconductors, most of them based on the AndersonKim theory, prove to be a useful source of information relative to the nature of the pinning and the characteristic fields. The use of models based on thermal activation integrates into scaling the tail of pinning force and also substantiates the use of the irreversibility field H irr as scale field. Finally, the data on scaling law in the two class of superconductors are presented and discussed. © 2012 World Scientific Publishing Company.
Enculescu M.,National Institute of Materials Physics Bucharest
Physica B: Condensed Matter | Year: 2010
Potassium hydrogen phthalate (KAP) crystals doped with rhodamine 6G (Rh 6G) and polyvinylpyrrolidone (PVP) were grown by solution evaporation technique. Nucleation occurred without the use of seeds and optically transparent crystals were obtained. The grown crystals were characterized by XRD measurements performed on crystals and their powders, optical transmission and photoluminescence measurements. The influence of the dopants on the structural, morphological and optical properties of the KAP crystals was analysed. Dopants do not change the structure of the single crystals while the addition of PVP changes the morphology of crystals from pseudo-hexagonal to rhomb. Three new XRD reflections are observed in all single-crystal and powder XRD spectra and are probably (0 3 0), (0 4 0) and (0 5 0) lines. The UV cut-off and transparency of the crystals are not changed by doping. Dye-doped KAP crystals exhibit a strong emission band centred at 550 nm excited with 480 nm wavelength. For the dye-doped crystals the up-conversion was investigated and its second harmonic origins are proved using photoluminescence measurements. © 2010 Elsevier B.V. All rights reserved.
Polosan S.,National Institute of Materials Physics Bucharest
Materials Research Bulletin | Year: 2010
Theoretical and experimental determinations show that the intrinsic luminescence of BGO materials is strongly influenced by GeO4 - tetrahedra around Bi3+ ions. Besides Bi3+ transitions, the excitonic spectra mask the BGO bandgap. Theoretical computations give the bandgap at 5.19 eV compared with the experimental one at 5.17 eV in BGO crystals and a significant decrease at 3.26 eV in the amorphous materials. This is due to the high disordering of the GeO4 - tetrahedral which also reduces the refractive index of the amorphous materials. Formation of excitonic states and their overlap with the Bi3+ transitions suggests that the energy absorption takes place via the p-electrons of the O2 - ions and is then transferred to the Bi3+ p-electrons (excited states) close to the conduction band. Light emission appears after de-excitation to the ground state. © 2010 Elsevier Ltd. All rights reserved.